Security document having a security element embedded therein with visually and machine-testable marks

ABSTRACT

In a security document having an embedded security element in the form of a transparent thread with electrically conductive material in at least two layers, at least one layer is transparent or partly transparent at least in certain areas. The partly transparent layer interacts with marks located on the thread in such a way that the marks are largely concealed in incident light but are recognizable visually in transmitted light.

The present invention relates to a security document having a securityelement in the form of a transparent thread or strip embedded thereinbearing visually recognizable marks in the form of characters, printedpatterns and the like and designed so as to be electrically conductivefor machine-testability.

It is known to protect security documents by embedding therein securityelements that exhibit particular machine-detectable physical propertiesand/or have a visual appearance that permits them to serve asauthenticity features for the security document. These security elementsare e.g. threads or strips that are embedded directly in the paper layeras it is forming in the course of the paper production.

For example, German laid-open print no. 14 46 851 discloses asafeguarding thread provided on both sides with microprints. To allowfor the patterns printed on the front and the back to be checkedindependently of each other an aluminum layer is placed therebetween.This aluminum layer can also serve as a machine-readable feature if itselectric conductivity is determined. In practice this form has notproved to be very useful since the aluminum layer makes the safeguardingthread opaque and the writing can only be recognized in incident lightand even then only with great difficulty. It is usually necessary tomake the paper transparent by chemical means at least for the time oftesting. Furthermore, the microprint of the safeguarding thread overlapsthe outer printed pattern, which is likewise felt to be disturbing inmany cases.

Since safeguarding threads can only be embedded up to a certain widthwithout holes during sheet forming, it has been proposed to incorporateporous threads, which can be of accordingly wider design (Germanlaid-open print no. 21 52 090). In a special embodiment a safeguardingthread is also described that shows different colors when viewed inincident light and in transmitted light. The thread is provided for thispurpose with two color layers having a semitransparent aluminum coatingtherebetween. When this thread is viewed in incident light after it isembedded in the paper, the color layer located above the reflectivealuminum coating is dominant, while in transmitted light the secondarycolor from the two colors will be the dominant color. However, suchthreads involve adhesion problems between the color layers and the metallayer located therebetween, whereby the outer color layer can even bedetached from the metal layer. Although this thread is thus basicallymachine-testable with respect to its electric conductivity, this featureis not a reliable authenticity feature due to the lack of durability ofthe metal coating over the total length of the thread. Cracks or partialdetachment of the metal layer will already cause an interruption in theelectric conductivity. Such security documents are accordinglyclassified as forgeries in during machine testing although they areauthentic.

A safeguarding thread having very easily tested writing after beingembedded in paper is known e.g. from U.S. Pat. No. 4,652,015. Thisthread comprises a transparent carrier having printing in the form of aplurality of single shiny microcharacters. The known safeguarding threadand the microcharacters located thereon are not recognizable in incidentlight. In transmitted light, on the other hand, solely the charactersare visible as sharply contoured marks since the carrier itself isdesigned so as to be transparent. Such a safeguarding thread is producedby metalizing a transparent film with an aluminum layer over a largesurface, printing the microcharacters on this layer using an acidproofink, and then etching off the unprinted areas, leaving the characters assingle characters separated from one other on a transparent background.

This thread accordingly has an easily recognized visual feature, butloses the property of electric conductivity due to the split into singlecharacters and the resulting interruption in the metal layer. It is thusunsuitable for machine testing.

Non-prepublished German patent no. 38 07 126 describes a safeguardingthread having a metal coating with characters worked into it in anegative form. When the security document is viewed in transmitted lightthe characters are visible as light marks on a dark background. Thisform of presentation makes it possible to produce the safeguardingthread with an uninterrupted metal coating so that the electricconductivity is in principle maintained over the entire length of thethread. The recesses in the metal layer necessary for representing thecharacters, however, increase the probability of this metal layer beingcracked by frequent bending and use of the security document, which nolonger permits the electric conductivity to be tested.

The invention is based on the problem of providing a security element inthe form of a thread or strip suitable for embedding in a securitydocument and bearing an easily recognized visual mark in the form ofwriting or a printed pattern, etc., the safeguarding thread additionallyhaving electrically conductive properties as a machine-readable featureand this property being reliably and clearly detectable even afterstrong mechanical stressing of the security document.

This problem is solved by the features stated in the characterizing partof the main claim.

In known safeguarding threads containing only one electricallyconductive layer, cracks in the metal coating generally lead to aninterruption in the conductivity so that it is no longer testable. Inthe preferred embodiment of the inventive solution there are now twoconductive layers that are capacitively coupled by a nonconductive layer(plastics film, adhesive layer, etc.) forming a dielectric. Theconductivity of such threads is tested according to the invention not bygalvanically contacting the electrically conductive layers, but bycapacitively coupling in high-frequency signals. If cracks occur in themetal layers, the metal layers interrupted by these cracks, with thedielectric therebetween, act as series or parallel connected capacitorsdepending on the test setup. This maintains an a.c. conductivity thatpermits interrupted layers to be tested. In particular when the twometal layers are not fully interrupted at the same place in the thread.If this a.c. conductivity is measured by known capacitive methods ofmeasurement, as described for example in German laid-open print no. 3843 077 or German patent no. 17 74 290, the presence of two electricallyconductive layers can be deduced even if there are one or more rupturesin one or both metal coatings, so that this feature indicatingauthenticity can still be clearly detected.

One of these electrically conductive layers may be an opaque metalcoating that, according to the application described in DE-P 38 07 126,contains recesses in the form of characters. The second electricallyconductive layer is preferably a transparent, electrically conductiveITO layer (indium tin oxide) applied, for example by the sputteringtechnique, to the same side of the synthetic thread or to the oppositeside. Since the second electrically conductive layer is transparent, theprinting in the first opaque metal layer is unchanged in its goodvisibility.

Instead of sputtering on transparent layers, which is more expensive interms of materials processing, one can also evaporate on appropriatelythin aluminum layers, which when applied e.g. in an amount of approx. 20mg/m² exhibit sufficient surface conductivity and are sufficientlytransparent for the negative writing to be readily visible at least intransmitted light.

But also other safeguarding threads, e.g. printed safeguarding threads,can be made electrically conductive by this method without substantiallyimpairing the visual testability of the printed pattern. In a preferredembodiment, the safeguarding thread comprises a transparent carrier filmprovided on one or both sides with a printed pattern or writing, anopaque ink being used for producing this print. This print is providedon both sides with a partly transparent metal coating which gives thethread its electric conductivity. Furthermore, the partly transparentmetal coating, due to its reflection properties, causes the print on thesafeguarding thread not to appear, or hardly to appear, when thesecurity document is viewed in incident light. There is therefore nopossibly disturbing overlapping between the outer print on the securitydocument and the print on the safeguarding thread embedded in thesecurity document.

A further embodiment of the partly transparent layer comprises the useof a conductive plastics material. Such plastics materials are generallynot supertransparent, like photoconductive materials, but partlytransparent so that they can be combined inventively with otherproperties detectable in transparency.

In a further development of the invention, properties of a safeguardingthread that are visually recognizable in transmitted light are alsotestable by machine. During an attempt at machine detection, visuallyvery striking properties of a thread, such as its color or itsmicroprint, are lost in the colored getups of the safeguarding threadcustomarily contained in the security document. However, if thesafeguarding thread is equipped according to the invention with partlytransparent conductive material this can be used to "trigger" themeasurement of the other properties. That is, the measurement, e.g. ofcolor or of structured prints, takes place precisely at the place wherethe conductivity occurs. In this way, coincidence measurement ofconductivity and other properties of the safeguarding threadrecognizable in transmitted light make it possible to associate themreliably.

A further embodiment of the invention relates to the machine detectionof transparency properties of the safeguarding thread which are notdetectable visually, e.g. the infrared absorption. The infraredabsorption of the thread is lost, as described above, in the diverseinfrared absorption properties of the security document if themeasurement cannot be precisely associated with the location of thesafeguarding thread. According to the invention the conductivity of thesafeguarding thread helps to reliably trigger the measurement of theother properties here, too.

With known safeguarding threads characterized by a printed patternvisible in transmitted light, the easily machine-testable feature ofelectric conductivity had to be wholly dispensed with up to now in thecase of positive writing, while this property was at least highly proneto failure in the case of negative writing due to the mechanical loads,thereby reducing its value for automatic authenticity testing.

Since even interrupted metal layers exhibit a.c. conductivity, at leastone or more of the conductive layers can also be designed as screens,such as line, hole or cross screens, with possibly opaque screenelements. The degree of transparency of these conductive layers can bevaried when using opaque screen elements by the structural size of theseelements (line width, etc.) and the surface area covered (screenintervals, etc.).

The present invention makes it possible to provide safeguarding threadswith any desired characters, printed patterns, etc., without impairingthe electric conductivity. The conductivity as such is still measurableafter relatively high loads or even if there are interruptions, whichincreases their fail-safety and their value for automated testingsystems. This safeguarding thread is not only machine testable but alsohas an appearance varying in incident light and in transmitted lightthat cannot be imitated by alternative measures. The change inappearance is thus utilizable as a visual feature that also allows forauthenticity testing without the use of any aids. If a paper of valueequipped with the inventive safeguarding thread is reproduced usingcommercial copying apparatus, in particular color copiers, neither themachine-testable properties nor the readily visible properties arereproducible. The stated safeguarding thread thus also provideseffective copying protection for papers of value.

Further advantages and advantageous developments will emerge from thedescription of exemplary embodiments of the invention with reference tothe figures, in which

FIG. 1 shows a safeguarding thread with negative writing in a metallayer,

FIG. 2 shows a safeguarding thread with positive metal writing andadditional electrically conductive coatings,

FIG. 3 shows a safeguarding thread with a printed pattern andelectrically conductive layers,

FIG. 4 shows a safeguarding thread constructed of two films.

FIG. 1 shows a safeguarding thread 1 usable in antifalsification papersthat comprises a plastics strip 2 made of a tear-resistant plasticsmaterial such as polyester, and having on one surface an opaque coating3. This coating is preferably a reflective metal layer, e.g. an aluminumlayer, that has recesses 4 in the form of the characters and patterns tobe applied to the safeguarding thread. After a thread is embedded in anantifalsification paper this coating is invisible in incident lightsince the light reflected by the metal surface is diffusely scatteredagain in the paper. The recesses in the opaque layer are thus onlyrecognizable as light surfaces when viewed in transmitted light. Theback of the plastics film bears an electrically conductive layer 5 thatis preferably transparent, but at least partly transparent. Transparent,electrically conductive layers are e.g. indium-tin oxide layers that areapplied to the film material by the sputtering technique. The vacuumcoating of plastics films with such materials is known (Kunststoffe 78(1988) 9, G. Biekehor, Hanau "Vakuumbeschichten von Kunststoff-Folien,"pp. 763-765). In many cases, however, a "thin" aluminizing suffices,which is cheaper and simpler in terms of process technology. Aluminizingin an amount of approx. 20 mg/m² or less is characterized bysufficiently high transparency for most cases of application.

FIG. 2 shows a safeguarding thread 11 having writing consisting ofreflective metallic characters 14, as known for example from EP-A 0 279880. Since the single characters are not interconnected the thread isnot electrically conductive along its entire length if it only containsthese characters. According to the invention, this thread issupplemented by an electrically conductive layer 15 that extends acrossthe entire thread surface but is at least partly transparent, so as notto impair the visibility of the writing. A transmission factor ofapprox. 50% is generally sufficient to allow the writing to be clearlydetected without the use of aids even when the thread is embedded inpaper. If higher demands are to be met, one can also apply fullytransparent electrically conductive layers, such as the above-mentionedsputtered indium-tin oxide layer with e.g. 200 ohms per square surface.In the safeguarding thread shown in FIG. 2 characters 14 constitute oneconductive layer, and layer 15 the other. Layer 14 thus corresponds to alayer with many selectively introduced interruptions. To improve thecapacitive measurability the characters should cover as large an area aspossible. If the individual characters are connected with each other onewill likewise have an uninterrupted conductivity, as in the exampleshown in FIG. 1.

Instead of working the characters into a metal coating one can alsoprint plastics carrier 22 accordingly for safeguarding thread 21 (FIG.3). Print 24 can contain characters and/or colored patterns. Charactersare preferably applied using an opaque ink so that they are wellrecognizable as dark areas in transmitted light. For color patterns, onthe other hand, one preferably uses translucent or transparent colors sothat these patterns are poorly visible in incident light and arerecognizable as colored surfaces only in transmitted light. To give thisthread the property of electric conductivity one provides it withtransparent or translucent electrically conductive layers on both sides.If the print for this thread is not to compete with the outer print ofthe security document when the security document is viewed in incidentlight, one preferably uses partly reflective aluminizing, e.g. ofapprox. 20 mg/m² or 800 ohms per square surface, for the electricallyconductive layers. Since a surface conductivity of 10,000 ohms persquare surface suffices for machine testing, the aluminum layer can alsobe made substantially thinner if necessary.

In a special embodiment (FIG. 4), safeguarding thread 31 is composed oftwo carrier films 32 which enclose a printed pattern and/or electricallyconductive coatings 35a, 35b. This protects the thin and generallysensitive layers (ITO layer, aluminum layer, etc.) against abrasion. Thetwo films are interconnected by a nonconductive adhesive layer 36. Aparticular advantage of this structure is its symmetry. Such asafeguarding thread, that is unwound from a roll during paper productionand guided to the mold at a predetermined place in the pulp, is lesslikely to form "festoons" or twist after it is unwound from the rollthan asymmetrically constructed threads. This avoids the rejectsoccurring with threads of asymmetrical structure due to a lack offlatness of the thread in the paper layer.

Since the sensitive layers are moved to the inner areas of thesafeguarding thread one can now use materials that could virtually notbe used in the prior art due to the exposed position and the lack ofenvironmental stability.

We claim:
 1. A security document having a security element in the formof a transparent thread or strip embedded therein bearing marks in theform of characters, printed patterns and the like that are largelyconcealed when viewed in incident light but are optically detectablevisually or by machine in transmitted light, said transparent thread orstrip including at least one electrically conductive layer providingelectrical conductivity for machine testability, the improvement whereinthe electrically conductive layer is made of a material that is at leastpartially transparent and is disposed in the optical viewing path forthe marks when the marks are viewed in transmitted light.
 2. Thesecurity document according to claim 1, including an additionalconductive layer comprising an opaque metal material, said opaque metalmaterial arranged to define said marks.
 3. The security documentaccording to claim 1, wherein in addition to the at least partiallytransparent conductive layer, at least one further conductive layer isprovided and said electrically conductive layers are separatedgalvanically by an insulating layer.
 4. The security document accordingto claim 3, wherein the at least partially transparent conductive layercomprises a coating applied by sputtering.
 5. The security documentaccording to claim 4, wherein the at least partially transparentconductive layer comprises an indium-tin oxide layer.
 6. The securitydocument according to claim 3, wherein the at least partiallytransparent conductive layer comprises a vacuum evacuated metal layer.7. The security document according to claim 6, wherein said vacuumevaporated metal layer comprises aluminum.
 8. The security documentaccording to claim 7, wherein the aluminum has an equivalent thicknessof 20 mg/m².
 9. The security document according to claim 1, wherein theat least partially transparent conductive layer comprises a screen. 10.The security document according to claim 3, wherein the at leastpartially transparent conductive layer comprises a screen.
 11. Thesecurity document according to claim 1, wherein said marks are appliedby printing techniques.
 12. The security document according to claim 11,wherein the at least partially transparent conductive layer extends overeach of said marks.
 13. The security document according to claim 1,including an additional electrically conductive layer made of a materialthat is at least partially transparent and which is also disposed in theoptical viewing path for the marks when they are viewed in transmittedlight.
 14. The security document according to claim 13, wherein themarks are applied by printing techniques, and wherein the at leastpartially transparent conductive layers extend over the security elementon opposite sides of each of said marks.
 15. The security documentaccording to claim 14, wherein said at least partially transparentconductive layers are separated galvanically by an insulating layer. 16.The security document according to claim 15, wherein the insulatinglayer is made of aluminum oxide.
 17. A security document having asecurity element in the form of a transparent thread or strip embeddedtherein bearing marks in the form of characters, printed patterns andthe like that are largely concealed when viewed in incident light butare optically detectable visually or by machine in transmitted light,said transparent thread or strip comprising a pair of carrier films eachhaving an electrically conductive layer secured thereto, one of theconductive layers being formed of a material that is at least partiallytransparent and which is disposed in the optical viewing path for themarks when the marks are viewed in transmitted light and the otherconductive layer being arranged to define said marks for opticaldetection in transmitted light; said carrier films being adhered to eachother by a nonconductive adhesive layer.
 18. The security documentaccording to claim 17, wherein said electrically conductive layers aredisposed between said carrier films.
 19. The security document accordingto claim 2, wherein one of said conductive layer defining said markscomprises an opaque metal layer defining said marks as recesses withinsaid opaque metal layer.
 20. A security document having a securityelement in the form of a transparent thread or strip embedded thereinbearing marks in the form of characters, printed patterns and the likethat are largely concealed when viewed in incident light but areoptically detectable visually or by machine in tranmitted light, saidtransparent thread or strip comprising a pair of carrier films eachincluding an electrically conductive layer formed of a material that isat least partially transparent and which is disposed in the opticalviewing path for the marks when the marks are viewed in transmittedlight; said carrier films being adhered to each other by a nonconductiveadhesive layer.
 21. The security document according to claim 20, whereinsaid electrically conductive layers are disposed between said carrierfilms.
 22. A method for testing security documents containing at leastone security element in the form of a transparent thread or stripembedded in said security document, said security element bearing marksin the forms of characters, printed patterns and the like that aresubstantially concealed when viewed in incident light but are opticallydetectable visually or by machine in transmitted light, and wherein saidsecurity elements include an electrically conductive layer capable ofbeing detected electrically by machine, including the steps of:(a)electrically detecting the presence and location of the security elementin the security document; and (b) after locating the security element inaccordance with step (a), viewing the security element optically byviewing same in incident light.
 23. A method for testing securitydocuments containing at least one security element in the form of atransparent thread or strip embedded in said security document, saidsecurity element bearing marks in the form of characters, printedpatterns and the like that are substantially concealed when viewed inincident light but are optically detectable visually or by machine intransmitted light, and wherein said security elements include anelectrically conductive layer capable of being detected electrically bymachine, including the steps of:(a) electrically detecting the presenceand location of the security element in the security document; and (b)after locating the security element in accordance with step (a), viewingthe security element optically by viewing same in transmitted light. 24.A method for testing security documents containing at least one securityelement in the form of a transparent thread or strip embedded in saidsecurity document, said security element bearing marks in the form ofcharacters, printed patterns and the like that are substantiallyconcealed when viewed in incident light but are optically detectablevisually or by machine in transmitted light, and wherein said securityelements include an electrically conductive layer capable of beingdetected electrically by machine, including the steps of:(a)electrically detecting the presence and location of the security elementin the security document using galvanic conductivity of the securityelement; and (b) after locating the security element in accordance withstep (a), viewing the security element optically by viewing same inincident light.
 25. A method for testing security documents containingat least one security element in the form of a transparent thread orstrip embedded in said security document, said security element bearingmarks in the form of characters, printed patterns and the like that aresubstantially concealed when viewed in incident light but are opticallydetectable visually or by machine in transmitted light, and wherein saidsecurity elements include an electrically conductive layer capable ofbeing detected electrically by machine, including the steps of:(a)electrically detecting the presence and location of the security elementin the security document using galvanic conductivity of the securityelement; and (b) after locating the security element in accordance withstep (a), viewing the security element optically by viewing same intransmitted light.
 26. The process according to claim 22, wherein saidelectrical detection of the presence and location of the securityelement is carried out by a procedure other than utilization of galvanicconductivity of the security element.
 27. The process according to claim23, wherein said electrical detection of the presence and location ofthe security element is carried out by a procedure other thanutilization of galvanic conductivity of the security element.